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Physical Review Research

Spin-orbit and angle-resolved resonant attosecond photoionization delays

Dhruba J. Biswas, J. K. Wood, Nisnat Chakraborty, Anatoli S. Kheifets, Arvinder S. Sandhu

Published Mar 22, 2026• 1 citation
Publisher

Abstract

Understanding how spin-orbit coupling shapes ultrafast electron dynamics remains a central challenge across atomic, molecular, and condensed-matter systems. Here, we demonstrate that spin-orbit splitting in krypton naturally separates two distinct regimes of attosecond interferometry: a conventional above-threshold process involving interference between continuum pathways and an under-threshold regime in which discrete and continuum intermediate states coexist and interfere. Using spin-orbit and angle-resolved measurements with attosecond precision, supported by time-dependent simulations, we reveal that the under-threshold channel exhibits dramatic phase and delay variations—up to several hundred attoseconds—highly sensitive to photon energy and emission direction. In contrast, the conventional above-threshold channel shows smooth, featureless behavior. This duality exposes the strong influence of metastable resonances, establishing spin-orbit splitting as a built-in control knob for accessing structured continua. Beyond krypton, our results provide a broadly applicable framework for probing and controlling coupled spin, orbital, and many-body correlation dynamics at the attosecond timescale.

DOI
10.1103/qfch-b1cv

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The ability to control quantum correlations in strongly driven systems is a central challenge across quantum science, with implications for ultrafast dynamics, quantum control, and information processing. In photoionization, the emitted electron and residual ion may form an entangled system whose correlations encode the underlying light-matter interaction, yet control of their generation and observable manifestation in continuum systems remains largely unexplored. Here we demonstrate phase-resolved control of electron-ion correlations using phase-locked pulse sequences in the strong-coupling regime. We show that entanglement can be halted and reshaped with attosecond precision, and that phase-dependent correlations can be redistributed into population-based correlations, leading to entanglement that is directly reflected in joint observables. These results establish a route to coherently shape entanglement in photoionization and open new possibilities for accessing and controlling quantum correlations in systems where measurements are intrinsically basis constrained.
Attosecond chronoscopy represents a major breakthrough in the study of ultrafast phenomena and has the potential to revolutionize our understanding of the fundamental physics of matter. We theoretically investigate the spin-orientation-resolved attosecond chronoscopy for the first time by the circular RABBIT technique for Kr atoms. Due to the spin-orbit interaction and the sensitivity of ionization in circularly polarized fields to the sense of electron rotation in the initial state, the spin-resolved ionization rates of photoelectrons emitted from 2P1/2^2P_{1/2}2P1/2​ and 2P3/2^2P_{3/2}2P3/2​ channels can be expressed via mmm-resolved ionization rates of initial state, where mmm is the orbital magnetic quantum number. We demonstrate that the yields difference between spin-up and spin-down photoelectrons from each channel are closely associated with the different behaviors of corresponding Wigner time delay. We find that the Wigner time delay between spin-up and spin-down photoelectrons in the polarization plane can reach several tens of attoseconds in the co-rotating geometry, but a few attoseconds in the counter-rotating geometry. Our approach opens up a new avenue for probing the spin-dependent behavior of Wigner time delay, and lays the foundation for spin-orientation-resolved attosecond chronoscopy, which can be verified by the current experimental techniques.
Strong field ionization injects a transient vacancy in the atom which is entangled to the outgoing photoelectron. When the electron is finally detached, the ion is populated at different excited states with part of coherence information lost. The preserved coherence of matter after interacting with intense short pulses has important consequences on the subsequent nonequilibrium evolution and energy relaxation. Here we employ attosecond transient absorption spectroscopy to measure the time-delay of resonant transitions of krypton vacancy during their creation. We have observed that the absorptions by the two spin-orbit split states are modulated at different paces when varying the time-delay between the near-infrared pumping pulse and the attosecond probing pulse. It is shown that the coupling of the ions with the remaining field leads to a suppression of ionic coherence. Comparison between theory and experiments uncovers that coherent Raman coupling induces time-delay between the resonant absorptions, which provides insight into laser-ion interactions enriching attosecond chronoscopy.